Toxin composition of three species of marine gastropods Zidona angulata, Adelomedon brasiliana and Pachycymbiola brasiliana and the mussel Mytilus edulis, and an Alexandrium tamarense isolate (AMI)from the Mar del Plata coast, were determined by means of a sensitive and specific HPLC technique. Cultures of A. tamarense contain N-sulfocarbamoyl derivatives as major toxins. The next most abundant components were GTX1 neoSTX and GTX4. As minor components, unusual decarbamoyl toxins were detected. Saxitoxin was the lowest of the 10 detectable toxins. Comparison of toxins profiles revealed significant differences between the toxic composition of the dinoflagellate and that accumulated in the tissues of their primary and secondary consumers. In the mussel M. edulis, gonyautoxins comprise more than 95 o/o of total toxins and STX was the lower of the 8 detectable toxins. The scarcity of gonyautoxins in comparison with high STX abundance, is characteristic of snails.